Benignuman: I didn’t actually look these sources up myself. I copied them straight out of Rav Ellinson’s seifer. Yes, I know that’s not the most scholarly way to do things, but I figured it would suffice for the Coffee Room. And I do tend to trust Rav Ellinson.
I have seen the Misha Berurah inside and that is really the only source on that topic that matters. As you yourself acknowledged, the Mishna Berurah says that women have a chiyuv to cover their legs until and including the knee (irrespective of minhag hamakom, unlike the bottom part of the leg below the knee).
I brought the Mishna Berurah to show that you were incorrect in stating that it is muttar for the knees and above to show in communities where this is customary.